1. Field of the Invention
The present invention relates to a digital fuzzy inference apparatus for performing fuzzy inference and, more particularly, to a digital fuzzy inference apparatus for performing fuzzy inference by time-divisional control.
2. Description of the Related Art
The fuzzy theory was suggested by L. A. Zadeh, Professor at California State University, in 1965, and the possibility of practical use of the theory was proven by E. H. Mamdani, Professor at the University of London, in 1974. Various means of implementing the theory have been proposed afterward. There are the following typical examples of such means.
For example, Published Unexamined Japanese Patent Application No. 58-192407 discloses an operation control system for vehicles which lessens the frequency of notch-changes by inference using software. Published Unexamined Japanese Patent Application No. 61-20428 discloses analog fuzzy logic circuits implemented by current circuits.
Furthermore, in Nikkei Electronics, No. 457, Oct. 3, 1988, there are described processors using memories developed at Hosei University, North Carolina State University and so on, and processors dedicated to fuzzy controllers for writing data for inference into instruction memories, which are developed at Togai Infralogic Company.
The conventional fuzzy inference systems have the following drawbacks. The system based on software can be implemented for the time being by means of a personal computer, microcomputer or the like, but it is very slow in inference speed. The analog system using current circuits requires an interface for use in cooperation with a digital computer. The system using memories and the dedicated processor system require large-scale development tools.
Prior to a description of the drawbacks of a fuzzy inference circuit which is designed to be formed into an IC (integrated circuit), an outline of the fuzzy inference will be described first. The fuzzy inference is a type of inference which uses fuzzy rules (fuzzy inference rules) expressed by ambiguous (fuzzy) words that human beings employ in their every day life. The fuzzy rules can be described such that "if A=BIG and B=NORMAL then X=SMALL".
FIG. 1 is a block diagram showing a fuzzy inference circuit in which an if-part "if A=BIG and B=NORMAL" and a then-part "X=SMALL" are expressed by a fuzzy rule as the minimum unit. Referring to FIG. 1, reference numerals 1 and 2 respectively denote if-part membership function defining circuits; 3, a minimum-value calculator; and 4, a then-part membership function defining circuit. Reference symbols A and B denote input variables; and X, an output variable.
FIG. 1 shows the fuzzy inference circuit corresponding to one rule. However, a plurality of rules are normally required. In addition, the fuzzy inference circuit includes the two if-part membership function defining circuits. However, the number of if-part membership function defining circuits is changed in accordance with the number of input variables. For example, FIG. 2 shows an arrangement for N rules (input variables : A, B), which includes a maximum-value calculator 6 and a center-of-gravity calculator 7. Referring to FIG. 2, reference numerals 5.sub.l to 5.sub.n denote fuzzy inference circuits identical to the circuit shown in FIG. 1. These circuits are arranged in units of rules.
The maximum-value calculator 6 synthesizes output variables X for the respective rules and obtains the maximum values thereof. The center-of-gravity calculator 7 calculates the center of gravity from the respective maximum values.
When the circuit size of hardware required for one fuzzy inference is considered, a basic arrangement includes N rules, N maximum-value calculators, and one center-of-gravity calculator. That is, an increase/decrease in number of rules directly influences the size of a fuzzy inference circuit. Therefore, the following drawbacks are posed in a fuzzy inference circuit which is designed to be formed into an IC. Since fuzzy inference generally requires a plurality of rules, the circuit size is greatly increased. In addition, if the number of rules is increased due to the modification of a system, addition of circuits cannot be easily performed.